The present invention relates to metal nanocrystals and the synthesis thereof, and more particularly to soluble metal nanocrystals and a scalable synthesis therefor.
Metal nanocrystals are widely used as catalysts, and are increasingly being explored as single electron devices, self-assembled monolayers, and thin film precursors. A limitation on the applications of metal nanocrystals is the impracticality of scaling existing syntheses to an industrial scale. Cluster deposition in vacuum is characterized by low throughput, difficulty in stabilizing nanocrystals towards coalescence, and complex synthetic systems. (P. Jensen, Reviews of Modern Physics, 71(5), (1999) 1695-1734). Colloidal syntheses of metal nanocrystals are well established, yet suffer from low yields per volume and difficulty in removing colloidal stabilizers after synthesis. An early preparation for colloidal metal includes combination of a dilute solution of hydrazine hydrate (1:2000) with an ammoninical copper sulfite solution (1:1000) in the presence of gum arabicum. Careful heating yields a hydrosol which after four days of dialysis against distilled water yields a hydrosol that is red in reflection and blue in optical transmission (A. Gutbeir, G. Hoffineyer, Z. Anorg. Allgem. Chem., 44, (1905) 227). Other colloidal syntheses have developed that retain the characteristics of low concentration and a polymeric or surfactant stabilizer. (H. H. Huang et al., Langmuir, 13 (1997) 172-175; I. Lisiecki and M. P. Pileni, J. Phys. Chem., 99 (14) (1995) 5077-5082; and Nanoparticles and Nanostructured Films, J. H. Fendler, Wiley-VCH, 1998, Chapter 4.) Shuttle molecules have also been employed to transfer metal ions to an organic phase prior to reduction in the presence of a solubilizing passivating agent. While this method is attractive for producing metal ions from an otherwise acidic acid solution, the cost of shuttle molecules such as tetraalkyl ammonium salts is considerable. (Brust et al., J. Chem. Soc. Commun.. (1994) 801.) An additional group of metal nanocrystal syntheses has used an organic reducing agent as a ligand to complex a metal ion intended for reduction. While such methods produce good yields of metal nanoparticulate, such methods are characterized by particle agglomeration. (N. Arul Dhas et al., Chem. Mater. 10 (1998) 1446-1452.)
The ability to produce economically large quantities of metal nanocrystals that are soluble in a given solvent affords numerous opportunities to develop novel catalytic and materials systems. Thus, there exists a need for a metal nanocrystal synthesis that affords soluble nanocrystals by a process that is readily scalable to produce gram and kilogram quantities.
A process for forming metal nanocrystals includes the steps of complexing a metal ion and an organic ligand in a first solvent and introducing a reducing agent to reduce a plurality of metal ions to form the metal nanocrystal associated with the organic ligand. The organic ligand has the formula Axe2x80x94Lxe2x80x94(Q)n or 
where L is C1 to C30 alkyl, C5 to C30 cycloalkyl, C2 to C30 alkenyl, C6 to C30 cycloalkenyl, C6 to C40 aromatic; Q is a heteroatom containing moiety capable of coordinating a metal ion, the heteroatom including oxygen, nitrogen or sulfur; the heteroatom being present as an alcohol, carbonyl, carboxyl, phosphatidyl, sulfonyl, sulfinyl, nitrosyl, amino, imido, azide, thiol, ester, ether, secondary amino, thioester, thioether, silanol, siloxyl; and A is a solubility imparting moiety illustratively including hydrogen, alcohol, sulfonyl, sulfhydryl, amino, secondary amino, phosphatidyl, carboxyl, phenyl, nitro-, ester, ether, thioester and thioether; n is an integer between 1 and 4. A process for forming a copper containing crystal in particular includes the steps of forming a complex between a copper ion and an organic ligand in a solvent and introducing a reducing agent illustratively including hydrogen gas, hydrides and hydrazines to reduce the copper ions to form a copper nanocrystal associated with the organic ligand. Optionally, the ligand is chosen to impart solubility on the copper nanocrystal associated therewith in a second solvent immiscible with the reaction solvent so as to transport the copper nanocrystals into the second solvent and thereby leave the reaction byproducts in the reaction solvent. A solution includes a plurality of copper nanocrystals having an average domain size in between 1 and 50 nanometers, each nanocrystal having a surface passivated with an organic ligand having a molecular weight of less than 400 atomic units and a solvent having an affinity for a portion of the ligand extending from the copper nanocrystal surface.
The present invention provides a reductive synthesis of a metal ion complex where the metal ion complex ligands following reaction serve to prevent agglomeration and impart solubility to the resulting metal nanocrystal. The resulting metallic nanocrystals preferably form kinetically stable solutions, in contrast to suspensions.
As used herein, xe2x80x9cnanocrystalxe2x80x9d defines a crystalline domain having dimensions along at least one axis of between 1 nanometer and 100 nanometers.
As used herein, xe2x80x9csolubilityxe2x80x9d is defined as a substance dispersed in a liquid that is able to pass through a 0.2 micron filter and remain in the liquid for 24 hours after centrifugation at 7000 rpm for ten minutes.
A process for forming metallic nanocrystals according to the instant invention includes forming a complex between a metal ion and an organic ligand in a solvent. It is appreciated that the entire coordination sphere of the metal ion need not be filled by organic ligands; rather, spectator ions, solvent molecules and solvent ions may also form coordinate bonds to the metal ion. A reducing agent is then introduced to the metal ligand complex. The reducing agent is selected to have an electrochemical potential sufficient to reduce the metal ion from a positive oxidation state to a zero oxidation state metal atom or produce metal hydrides that in turn reduce to zero oxidation state metals. The result of metal ion reduction in the presence of the organic ligand-metal ion complex is the formation of a metal nanocrystal having associated therewith the organic ligand. The association of the organic ligand with the metal nanocrystal arrests nanocrystal growth, limits nanocrystal agglomeration, and preferably is selected to impart solubility on the resulting nanocrystal.
According to the present invention a metal nanocrystal is formed of a metallic element including beryllium, magnesium, aluminum, scandium, titanium, vanadium, chromium, manganese, iron, cobalt, nickel, copper, zinc, gallium, germanium, yttrium, zirconium, niobium, molybdenum, technetium, ruthenium, rhodium, palladium, silver, cadmium, indium, tin, antimony, lanthanum, cerium, praseodymium, neodymium, promethium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, lutetium, hafnium, tantalum, tungsten, rhenium, osmium, iridium, platinum, gold, thallium, lead, bismuth, polonium, thorium, protactinium, uranium, neptunium, and plutonium. Typically, an inorganic metal salt is chosen as the source of metal ions for reduction to form a nanocrystal. The choice of metal ion counter anion largely being dictated by solubility and compatibility with the solvent. Metal ion counter anions operative herein illustratively include halides, such as fluoride, chloride, bromide and iodide; nitrate; phosphate; perchlorate; formate; acetate; borate; hydroxide; silicate; carbonate; sulfite; sulfate; nitrite; phosphite; hydrates thereof; and mixtures thereof.
It is appreciated that a plurality of different metal ions are reduced simultaneously so as to form a metal alloy or metal ion doped metal nanocrystal, provided the predominant metal ion reagent based on atomic percent is present as a metal ion ligand complex. A dopant metal ion typically is uncoordinated and reduced in concert with the predominant metal ion-ligand complex. The dopant metal typically being present from 0.05 to 49 atomic percent of the total metal content of a nanocrystal. Preferably, the dopant metal is present from 0.5 to 5 atomic percent of the total metal present.
The total metal ion is typically present at a concentration of 0.01 to 1 molar. The concentration of the metal ion is dictated by factors illustratively including metal ion-counter ion solubility in the solvent, and desired nanocrystal size.
The solvent chosen for the formation of the metal ion ligand complex is dictated not only by reagent solubility, but also the stability of a zero oxidation state metal nanocrystal in the solvent. Thus, while water and water containing organic solvents are operative in the synthesis of most metal nanocrystals, those metals that readily form stable hydroxides or oxides are better synthesized in anhydrous organic solvents such as methanol, isopropanol, higher alcohols, ethers, and the like. It is appreciated that degassing of solvents and the synthesis of metal nanocrystals often requires exclusion of oxygen by performing reactions under an inert atmosphere illustratively including nitrogen, argon, and helium.
A ligand operative in the instant invention is any organic species capable of forming a coordinate covalent bond to a given metal ion. Suitable ligands for forming a complex with a given metal ion are well known to the art as detailed in Chemistry of the Elements, 2nd Ed., 1997 by N. N. Greenwood and A. Earnshaw, Butterwerth-Heinemann, Oxford, UK; Principles and Applications of Organotransition Metal Chemistry, 1987 by J. Collman et al., University Science Books, Mill Valley, Calif.; and Organic Sequestering Agents, S. Chaberek and B. E. Martell, 1959, Wiley, N.Y. A ligand operative in the present invention has a formula Axe2x80x94Lxe2x80x94(Q)n where L is C1 to C30 alkyl, C5 to C30 cycloalkyl, C2 to C30 alkenyl, C6 to C30 cycloalkenyl, C6 to C40 aromatic; Q is a heteroatom containing moiety capable of coordinating a metal ion, the heteroatom including oxygen, nitrogen or sulfur; the heteroatom being present as an alcohol, carbonyl, carboxyl, phosphatidyl, sulfonyl, sulfinyl, nitrosyl, amino, imido, azide, thiol, ester, ether, secondary amino, thioester, thioether, silanol, siloxyl; and A is a solubility imparting moiety illustratively including hydrogen, alcohol, sulfonyl, sulfhydryl, amino, secondary amino, phosphatidyl, carboxyl, phenyl, nitro-, ester, ether, thioester and thioether; n is an integer between 1 and 4 and corresponding to mono through tetradentate ligands. In a preferred embodiment, L is alkyl. It is further appreciated that in each occurrence of Q in a polydentate ligand, Q is independently selected from the heteroatoms oxygen, nitrogen and sulfur. In a preferred embodiment, a polydentate ligand has at least two different heteroatoms Q. For example, a first occurrence of Q is an amine nitrogen and a second occurrence an alcohol oxygen leading to a preferential association of one heteroatom metal ion bond upon reduction. While the identity of a heteroatom containing coordinating group Q and a solubility imparting moiety A may in some instances be the same, A is differentiated in such an instance from Q in not being coordinated to a metal ion owing to steric effects. It is appreciated that the ligand may also be a cyclic 
incorporating the heteroatom containing group Q therein, where the identity of A, L, Q, and n are the same as provided above. Cyclic ligands illustratively include tetrahydrofuran, thiophene, pyridine, piperazine, lactones, diazoles, triazoles, thiazoles and sugars. A limitation on the ligand is that the resulting complex remain susceptible to reduction to a zero oxidation state metal atom. Preferably, the ligand has a molecular weight of less than 400 atomic units. The identity of Q to form successfully a ligand metal ion complex depends on the bonding characteristics of Q relative to the metal ion and the zero oxidation state metal atom. While the metal ion ligand complex must have a metal ion xe2x80x94Q bond weak enough to allow action of the reducing agent on the metal ion, Q must also have an affinity for zero oxidation state metal atoms on the nanocrystal surface. Bond strengths of Q and a metal ion or the zero oxidation state metal formed after ion reduction are readily obtained for a given ligand-metal ion system by ab initio calculations using GAUSSIAN(copyright) program, predicted from bond dissociation or mean bond energies derived from thermochemical data. (CRC Handbook of Chemistry and Physics, 76th Ed. (1995) 9-15-9-63).
It is appreciated that a multiple ligand system is operative herein with a first ligand selectively binding a metal ion whereas a second ligand preferentially associated with the zero oxidation state metal atoms arrayed on the nanocrystal surface after reduction. By way of example, many main group metal ions form operative amine complexes which are better stabilized as zero oxidation state metals by sulfur or oxygen. Specifically, a Cu2+ complex where Q is a primary amine includes a second ligand where Q is an alkyl thiol or thioester. The sulfur containing ligand displaces the amine in stabilizing the resulting copper nanocrystal. It is further appreciated that a multiple ligand system need not have all ligands soluble in the same solvent; rather, the metal ion complexing ligand while forming a stable complex with the metal ion only forms a transiently stable association with the resulting nanocrystal that imparts solubility properties on the nanocrystal to transport the nanocrystal into an immiscible second solvent layer containing the ultimate nanocrystal passivating second ligand that preferentially displaces the first metal ion ligand.
The ratio of metal ion to ligand molecules is dependent on factors illustratively including desired nanocrystal size, the value of n, the steric bulk of a ligand, and the ability of a metal-ligand bond to survive following metal ion reduction. The atomic ratio of metal ions to heteroatoms containing coordinate groups Q range from 1:0.10 to 1:6. It is appreciated that facile ligands in the synthesis of smaller size nanocrystals favor a high ratio of ligand to metal ions. For a monodentate ligand, a metal to ligand ratio is preferably between 1:0.5 and 1:4.
Often, upon introducing a ligand to a metal ion solution, the resulting complex is insoluble and forms a precipitate. Such a complex precipitate is equally operative herein with a soluble metal ion-ligand complex provided the precipitate is suspendable in solvent.
A reducing agent is added to a metal ion-ligand complex solution or suspension to reduce the metal ion to a zero oxidation state. The combining of a plurality of zero oxidation state metal ions associated with ligands affords the inventive metal nanocrystals. A reducing agent operative herein is a Hxe2x88x92 transfer reagent or an electrochemical potential greater than the reduction potential of the metal ion to be reduced. (CRC Handbook of Chemistry and Physics, 76th Ed. (1995) 8-26-8-31, CRC Press, Boca Raton, Fla. ). Generally, main group metal ions are readily reduced by water and alcohol soluble reducing agents illustratively including hydrazine, sodium borohydride and lithium aluminum hydride. Additionally, bubbling of a gaseous reducing agent through a suspension or solution of metal ion-ligand complex, or the addition of a reactive metal under anhydrous conditions is also operative herein. It is appreciated that reactive metal reducing agents offer a larger electrochemical potential needed to reduce some metal ions as per the electrochemical series. In instances where alternative reducing agents have the necessary electrochemical potential to drive a metal ion reduction, a solution of hydrazine or sodium borohydride is preferred. However, the preferred reducing agents are not compatible with acidic solutions and in such instances, hydrogen is the preferred reducing agent. More preferably, the reducing agent is added in excess to assure complete reduction of metal ions to zero valence metal.
While the size of metal nanocrystals formed can vary from 1 nanometer to 100 nanometers in size, the preferred nanocrystal domain size is between 1 nanometer and 8 nanometers to take advantage of the high surface area to volume ratio associated with particulate in this size regime. As such, metal nanocrystals as produced herein have utility as catalysts, precursors for thin films, single electron devices, sensors, biomedical markers, and in the case of radioactive metals, higher radiative flux fission sources.
In a preferred embodiment, a second solvent, immiscible with the metal ion solution, is added thereto. The second solvent is chosen such that the portion of the ligand molecule terminal to the metal nanocrystal when associated therewith is attractive to the solvent. For example, an alkyl chain extending from a heteroatom bound to a metal nanocrystal surface is attractive towards a lipophilic solvent such as an alkane, ether or toluene thereby imparting solubility on the passivated metal nanocrystal.
Having described the invention, the following illustrative examples are provided which are not intended to limit the scope of the invention.